If I'm paying by the day (instead of by the season), I avoid "ski a couple runs" sort of days. The first trip out west, I skied 4 days in a row, and felt that the 4th day, I was less-than-good. I was sore and stiff, and didn't really enjoy myself.
Last trip, I did 3 days, then took a day to go snowshoeing, then skied 2 more, and that worked well. I think after a week of "break in" time (coming from the East Coast), I could go more than 3-4 days at a time, but since I was paying by the day, I didn't want to spend a day being sloppy.
On the other hand, if I've got a season pass (or something close to it), I'd have no problem with going out for 3-4 runs and calling it quits (for either personal fatigue issues or condition suckage)...